1. Field of the Invention
This invention relates to a method of manufacturing a fuel cell, and in particular relates to a method of manufacturing a fuel cell wherein there is little statistical scatter of the output characteristics of the unit cells of a phosphoric acid fuel cell assembly.
2. Discussion of the Background
Fuel cells are already known and are devices whereby the energy possessed by fuel can be directly converted into electrical energy. Normally in such a fuel cell, a pair of porous electrodes are arranged on each side of an electrolyte-impregnated matrix. A fluid fuel such as hydrogen is brought into contact with the back face of one electrode, while an oxidizing agent in fluid form, such as oxygen, is brought into contact with the back face of the other electrode. An electrochemical reaction then occurs, and this is used to extract electrical energy from between the two electrodes. Electrical energy can be extracted with high conversion efficiency so long as fuel and oxidizing agent are supplied.
In particular, in the case of fuel cells which employ phosphoric acid as the electrolyte, a plurality of unit fuel cells are laminated to form a laminated fuel cell assembly.
In laminating such unit cells the output characteristics of the units cells must be the same as far as possible. The reason for this is as follows. If there is statistical scatter of the unit cell characteristics of a laminated fuel cell assembly consisting of laminated unit cells, this will produce a non-uniform current distribution between cells having poor characteristics and cells having good characteristics. This will produce a non-uniform temperature distribution among the cells, with abnormal local evolution of heat. This itself assists in producing a non-uniform current distribution, which in turn accelerates the development of the non-uniform temperature distribution.
Cells which are subject to this problem show a progressive deterioration of output characteristics due to a drop in the activity of the electrode catalyst and/or due to evaporation of electrolyte, etc. and ultimately become incapable of generating electricity.
In the case of a fuel assembly laminated of a plurality of unit cells, if even one of these unit cells is incapable of generating electricity, this makes the whole fuel cell assembly incapable of generation. For this reason, to ensure stable generation of electricity over a long period, it is necessary that the assembly should be built up of unit cells whose output characteristics are, as far as possible, the same.
Normally, fuel cell manufacture is carried out with strict quality control in order to reduce such statistical scatter of the unit cell characteristics to the utmost extent. Nevertheless, cells with poor characteristics are still sometimes manufactured.
Investigation of cells with poor output characteristics has revealed the following. In a cell comprising gaseous diffusion electrodes wherein a catalyst is supported on a porous substrate, a balanced so-called three phase zone, in which catalyst, electrolyte and reaction gas co-exist, performs a vital function in ensuring that the unit cell electrode reaction proceeds smoothly.
If the action of any one of the constituents of this three phase zone is either too strong or too weak, this will hinder the smooth running of the electrode reaction. For example:
(1) If the catalyst is thickly covered with electrolyte, the supply of reaction gas to the catalyst will be obstructed, causing a drop in output due to an increase in concentration overvoltage.
(2) If there is little contact between the catalyst and electrolyte, a decrease in reaction area and an increase in ionic resistance of the electrolyte, will occur leading to a drop in output due to increase in the resistance overvoltage.
(3) If the catalyst surface area is small or the activity of the catalyst is low, a drop in output due to an increase in the activation overvoltage will occur.
In actual fuel cells, these factors interact in a complex way to bring about a drop in output.
Keeping this three phase zone balanced over a long period of time is therefore vitally important for developing long-life unit cells.
Conventionally, in cells employing an aqueous solution of, for example, phosphoric acid or potassium hydroxide as the electrolyte, in order to maintain a balanced three phase zone, a fluoropolymer such as polytetrafluoroethylene (PTFE) was used as a hydrophobic agent to prevent gross leakages of catalyst.
It is possible to control the hydrophobic character of the catalyst to some extent by addition of such a hydrophobic agent, by controlling the amount added, the method of mixing or temperature of heat treatment etc. of this agent. However, in general, once the polytetrafluoroethylene-added catalyst has lost its hydrophobic character, it is difficult to recover its original hydrophobic character without again subjecting it to heat treatment, etc.
For this reason, in order to maintain the hydrophobic character of the catalyst for as long as possible, the catalyst is given a rather strong hydrophobic character when it is applied to the porous substrate. Statistical scatter in the degree to which such hydrophobic character is successfully applied results in statistical scatter of the characteristics of the three phase zone, in particular of the degree of contact of the catalyst and the electrolyte. This results in the manufacture of cells having poor characteristics.
Accordingly, the practice is not to operate such full cell assemblies (laminated from unit cells whose three phase zones are subject to such statistical scatter of their characteristics) at large current densities right from the start, but to perform a preparatory operation for several hours to several tens of hours at low current density. By carrying out this preparatory operation, the local development of excessive current concentrations can be prevented, thereby enabling local evolution of heat to be prevented and enabling aggravation of the statistical scatter of the unit cell characteristics to be controlled.
By continuing operation at low current density, movement of the electrolyte and permeation of reaction gas in the catalytic layer are stabilized. This enables a gradual increase in formation of the three phase zones and so reduces the scatter of the unit cell characteristics.
However, the characteristics of some cells do not stabilize even after several tens of hours but require several hundreds of hours to stabilize. And even then, some cells will be found not to have stablized.
It is therefore impractical to wait until the characteristics of such cells of low and unstable characteristics have finally stabilized before operating with high current as the rated load. Thus, in practice situations may arise in which operation of a fuel cell assembly for a long period at its rated load becomes impossible. Out of necessity, attempts have therefore been made to make it possible to achieve long-period operation by using the cell assembly under partial load. None of these measures, however, are to be recommended, as they involve increased equipment costs or operating costs.
Thus, as explained above, conventional statistical scatter of the characteristics of the unit cells of a fuel cell assembly cannot be avoided, and this is a factor which accelerates the drop in output characteristics and shortens the life of such fuel cell assemblies.
The cause of this statistical scatter in the characteristics of the unit cells is statistical scatter of the three phase zones that are active in the electrode reaction. The largest contribution to this scatter is the scatter of hydrophobic characteristics of the catalyst and electrolyte. In particular, cells whose hydrophobic characteristics are too strong present a considerable problem.